Make validation interface capable of signalling header presync

This makes a number of changes:
- Get rid of the verification_progress argument in the node interface
  NotifyHeaderTip (it was always 0.0).
- Instead of passing a CBlockIndex* in the UI interface's NotifyHeaderTip,
  send separate height, timestamp fields. This is becuase in headers presync,
  no actual CBlockIndex object is available.
- Add a bool presync argument to both of the above, to identify signals
  pertaining to the first headers sync phase.

src/node/interface_ui.cpp

@@ -53,7 +53,7 @@ void CClientUIInterface::NotifyNetworkActiveChanged(bool networkActive) { return
 void CClientUIInterface::NotifyAlertChanged() { return g_ui_signals.NotifyAlertChanged(); }
 void CClientUIInterface::ShowProgress(const std::string& title, int nProgress, bool resume_possible) { return g_ui_signals.ShowProgress(title, nProgress, resume_possible); }
 void CClientUIInterface::NotifyBlockTip(SynchronizationState s, const CBlockIndex* i) { return g_ui_signals.NotifyBlockTip(s, i); }
-void CClientUIInterface::NotifyHeaderTip(SynchronizationState s, const CBlockIndex* i) { return g_ui_signals.NotifyHeaderTip(s, i); }
+void CClientUIInterface::NotifyHeaderTip(SynchronizationState s, int64_t height, int64_t timestamp, bool presync) { return g_ui_signals.NotifyHeaderTip(s, height, timestamp, presync); }
 void CClientUIInterface::BannedListChanged() { return g_ui_signals.BannedListChanged(); }
 
 bool InitError(const bilingual_str& str)

src/node/interface_ui.h

@@ -105,7 +105,7 @@ public:
     ADD_SIGNALS_DECL_WRAPPER(NotifyBlockTip, void, SynchronizationState, const CBlockIndex*);
 
     /** Best header has changed */
-    ADD_SIGNALS_DECL_WRAPPER(NotifyHeaderTip, void, SynchronizationState, const CBlockIndex*);
+    ADD_SIGNALS_DECL_WRAPPER(NotifyHeaderTip, void, SynchronizationState, int64_t height, int64_t timestamp, bool presync);
 
     /** Banlist did change. */
     ADD_SIGNALS_DECL_WRAPPER(BannedListChanged, void, void);

src/node/interfaces.cpp

@@ -377,9 +377,8 @@ public:
     std::unique_ptr<Handler> handleNotifyHeaderTip(NotifyHeaderTipFn fn) override
     {
         return MakeHandler(
-            ::uiInterface.NotifyHeaderTip_connect([fn](SynchronizationState sync_state, const CBlockIndex* block) {
-                fn(sync_state, BlockTip{block->nHeight, block->GetBlockTime(), block->GetBlockHash()},
-                    /* verification progress is unused when a header was received */ 0);
+            ::uiInterface.NotifyHeaderTip_connect([fn](SynchronizationState sync_state, int64_t height, int64_t timestamp, bool presync) {
+                fn(sync_state, BlockTip{(int)height, timestamp, uint256{}}, presync);
             }));
     }
     NodeContext* context() override { return m_context; }